Merge pull request #7921 from dotty-staging/fix-#7851

Fix #7851: Add regression test

Author: odersky

tests/pos/i7851.scala

@@ -0,0 +1,37 @@
+trait Wrappable[T] { }
+given Wrappable[Float] { }
+
+case class Wrapped[T: Wrappable](value: T)
+
+trait Wrapper[T] { type WrappedT }
+object Wrapper { type Aux[T <: Tuple, WrappedT0 <: Tuple] = Wrapper[T] { type WrappedT = WrappedT0 } }
+
+given Wrapper[Unit] { type WrappedT = Unit }
+
+given [T: Wrappable]: Wrapper[T] { type WrappedT = Wrapped[T] }
+
+given [H: Wrappable, T <: Tuple, WrappedT0 <: Tuple](given Wrapper.Aux[T, WrappedT0]): Wrapper[H *: T] {
+  type WrappedT = Wrapped[H] *: WrappedT0
+}
+
+def wrappedFunction[F, FArgs <: Tuple, WrapperFArgs <: Tuple, R: Wrappable](
+  function: F
+)(input: FArgs)(given
+  tf: TupledFunction[F, WrapperFArgs => Wrapped[R]],
+  vs: Wrapper.Aux[FArgs, WrapperFArgs]
+): (R, R => Option[FArgs]) = {
+  val variableInput = input.asInstanceOf[WrapperFArgs] // This is not correct but it's ok for the sake of this example.
+  val result = tf.tupled(function)(variableInput)
+  return (result.value, (_: R) => None)
+}
+
+object WrapperTest {
+  def test1(x: Wrapped[Float], y: Wrapped[Float], z: Wrapped[Float]): Wrapped[Float] = { x }
+  val test2: (Wrapped[Float], Wrapped[Float], Wrapped[Float]) => Wrapped[Float] = { (x, y, z) => x }
+
+  def main(args: Array[String]): Unit = {
+    wrappedFunction(test1: (Wrapped[Float], Wrapped[Float], Wrapped[Float]) => Wrapped[Float])(5f, 11f, 3f)
+    wrappedFunction(test1)(5f, 11f, 3f)
+    wrappedFunction(test2)(5f, 11f, 3f)
+  }
+}